1. Field of the Invention
The present invention relates to a tunable-filter control method and apparatus, which are usable in communication systems such as wavelength division multiplexing (WDM) communication systems, and to an optical communication system which uses the tunable-filter control method or apparatus in a receiver unit.
2. Related Background Art
In recent years, as multimedia systems for data or other information have been developed, studies have been conducted regarding possible communication systems for implementing such multimedia systems. Optical communication systems, such as subscriber line systems and local area networks (LAN), are one type of system that has been studied. In particular, WDM communication is one such optical communication system, in which a large number of independent channels can be established in a single transmission line or optical fiber. WDM communication offers both the high-speed characteristics that are inherent in an optical communication system and flexibility of communication. Therefore, WDM communication has come into wide use as a communication system which transmits or exchanges high-speed information and which has a large capacity. As a component of such a WDM system, tunable or wavelength-changeable filters are widely known.
The tunable filter is an indispensable element for use in WDM optical communication systems, and performs a function of selecting a light signal at one wavelength from a plurality of wavelength-multiplexed light signals. In an optical communication system which utilizes a small number of wavelengths (e.g., several wavelengths), and which has a wide wavelength interval between channels, dielectric multi-layer fixed filters may be adopted as such a tunable filter.
On the other hand, in an optical communication system which utilizes a large number of wavelengths where the interval between multiplexed wavelengths is narrow, a filter is needed which can vary its filtering wavelength and which has a narrow transmission wavelength width. As such a kind of tunable filter, a filter having a distributed feedback laser diode (DFB-LD) structure, or a DFB filter, as it is commonly known. A DFB filter is disclosed, for example, in Preliminary Papers of Division Conference, Lecture No. 326, Japan Electronics Communication Academy, 1985. In DFB filters, a central wavelength in a transmission spectrum (also referred to as a central wavelength) can be controlled by changing a bias current of the filter in a range below a losing threshold. The DFB filter can serve as an optical amplifier in the vicinity of its central wavelength.
However, the central wavelength of a DFB filter is dependent on temperature, and on top of that, DFB filters each have different correlation characteristics between bias current and central wavelength. Therefore, when a DFB filter is used in a WDM communication system, a tracking control system is needed in which the central wavelength in the transmission spectrum of the DFB filter is drawn into a target received wavelength and stabilized thereat. A prior art tunable-filter control system will be described in the following.
This prior art is based on the disclosure in the periodical: "Electronics Letters", Vol. 26, No. 25, pp. 2122-2123, December, 1990. FIG. 1 shows this tunable filter control system. A light signal transmitted through a tunable filter 811 enters a light receiving element 813. The light signal is converted to an electric signal by the light receiving element 813, and the electric signal is amplified by an amplifier 814. Only a modulation component of the electric signal is further amplified by a narrow band amplifier 810 for amplifying only a frequency component near a frequency f, and the output of amplifier 810 is input to a synchronous detector or detection circuit 820. The synchronous detection circuit 820 detects the modulation component signal in synchronization with a modulation signal supplied from a sinusoidal wave generator or oscillator 819 through a delay circuit 825. Only a low frequency component of an output from the synchronous detection circuit 820 is amplified by an integrator 830. The time constant of the integrator 830 is set to a value sufficiently larger than the period of the above-mentioned modulation signal.
A feedback control circuit 817 generates a control signal by using the low frequency component as an error signal. A filter driver or control circuit 818 generates an output based on three input signals consisting of an offset signal from a receiver portion, the modulation signal from the sinusoidal wave oscillator 819 and the control signal from the feedback control circuit 817, and supplies the output to the tunable filter 811. Thus, the central wavelength in the transmission spectrum of the tunable filter 811 is controlled. The sinusoidal wave oscillator 819 generates the modulation signal, which is a sinusoidal wave of the frequency f, and supplies the modulation signal to both the filter driver 818 and the synchronous detection circuit 820.
FIG. 2 is a graph illustrating a relation between the transmission spectrum of the tunable filter 811 and a wavelength .lambda.s of a received signal. In FIG. 2, symbol .lambda.c designates the central wavelength of the transmission spectrum of the tunable filter 811. The central wavelength .lambda.c is drawn-in from left to right in the case of .lambda.c&lt;.lambda.s, and from right to left in the case of .lambda.c&gt;.lambda.s, by the tracking operation. A range, in which the draw-in is possible, is a range between locations that are slightly shifted toward a peak of the transmission spectrum of the tunable filter 811 from respective feet thereof. The range is determined by characteristics of the control system. This range will hereinafter be referred to as a draw-in range.
The central wavelength of the transmission spectrum of the tunable filter 811 is modulated (i.e., varied) by the sinusoidal modulation signal, from the sinusoidal oscillator 819, input into the filter driver 818. The modulation amplitude of the central wavelength is sufficiently smaller than the width of the transmission spectrum of the tunable filter 811. For example, the former is a value less than a tenth of the value of the latter. A change in the central wavelength .lambda.c of the transmission spectrum of the tunable filter 811 is converted to a change in the amount of light at the wavelength .lambda.s that is transmitted through the tunable filter 811, and is further converted to an electric signal by the light receiving element 813. The phase of a sinusoidal wave of a frequency f contained in that electric signal is opposite in the case of .lambda.c&lt;.lambda.s from the case of .lambda.c&gt;.lambda.s.
The low frequency component of the output from the synchronous detection circuit 820 (i.e., the output from the integrator 830) is positive when two inputs of the synchronous detection circuit 820 are in phase with each other, while negative when the phases of those two inputs are opposite to each other. When the above-mentioned electric signal and the output of the sinusoidal wave oscillator 819 are those two inputs, the polarity (positive or negative) of the output from the integrator 830 is opposite in the case of .lambda.c&lt;.lambda.s from the case of .lambda.c&gt;.lambda.s. The central wavelength .lambda.c of the transmission spectrum can be drawn-in to the wavelength .lambda.s of the received signal and stabilized thereat by performing feedback control based on the signal from the integrator 830, which serves as an error signal.
In the above-discussed prior art tunable-filter tracking control system, however, a periodic fluctuation of the central wavelength of the transmission spectrum due to the modulation by the modulation signal exists even after the central wavelength .lambda.c of the tunable filter 811 has been drawn-in to the wavelength .lambda.s of the received light signal (referred to as a stable state). The fluctuation of the central wavelength of the transmission spectrum will be converted to a fluctuation of an intensity of light which is incident on the light receiving element 813 through the transmission spectrum of the tunable filter 811. As a result, when the fluctuation of the light intensity is great, reception sensitivity will be degraded. In order to decrease the fluctuation of the light intensity, it would be necessary to reduce the modulation amplitude of the central wavelength .lambda.c of the tunable filter 811. In this case, however, the error signal at the draw-in time would lessen, and the response characteristics of the draw-in would be degraded.